The present invention relates to a copying apparatus and, more particularly to a transfer type electrophotographic copying apparatus with a movable platform to support an original to be copied.
Conventionally, in a copying apparatus of the above described type, there is usually employed a copying system known as the Carlson system, in which an electrostatic latent image formed on an electrophotosensitive photoreceptor in the configuration of a drum or an endless belt is subsequently visualized by applying thereto a developing material in the form of powder or liquid for the transfer of the visualized image onto a copy paper sheet, or another system wherein the latent image formed on the photoreceptor is directly transferred onto a copy paper sheet without visualization, with the latent image transferred onto the copy paper sheet subsequently developed for obtaining the copy of the original.
In the copying apparatus of the above described types, however, despite the long felt need for compact size and simple handling, there has been none which completely satisfies such needs.
In other words, in the conventional visualized image transfer type copying apparatus as described above, it is necessary to dispose various processing devices, such as a corona charger, an exposure device, a developing device, a transfer device, a copy paper sheet separating device, charge erasing devices and a cleaning device etc., around the photoreceptor, which arrangement inevitably results in a large size of the photoreceptor itself and complicated construction of the copying apparatus as a whole with consequent troublesome maintenance, while in the latter type system which develops the latent image transferred onto the copy paper sheet, no copying apparatus sufficiently fit for practical use has been developed as yet, though this latter type of copying apparatus can eliminate the disadvantages inherent in the former type since the developing device need not be located around the photoreceptor.
Furthermore, in the latter type of copying apparatus wherein the latent image formed on the photoreceptor is directly transferred onto the copy paper sheet, the copy sheet is fed in synchronization with the movement of the latent image on the photoreceptor drum, while the copy paper is pressed, at the transfer station, against the photoreceptor surface by an electrically conductive roller or a grounded roller so as to induce a charge having the same polarity as that of the latent image on an insulating layer of the copy paper sheet for transferring the latent image onto the copy paper sheet. In such case, if the copy paper sheet is directly fed between the photoreceptor surface of the photoreceptor drum and the grounded roller, a phenomenon similar to that in rapid generation of a strong electric field between the leading edge of the latent image formed portion on the photoreceptor drum surface and the grounded roller is observed, such a phenomenon giving rise to white spots or absence of black tone in the developed image due to undesirable electrical discharge between the photoreceptor drum surface and the reverse side of the copy paper sheet in a position, immediately before the copy paper sheet contacts the latent image formed portion on the photoreceptor surface, where the gap between the photoreceptor drum surface and the copy paper sheet is larger than the gap whereat the transfer is normally carried out. To eliminate the disadvantages as described above, there is conventionally proposed a method wherein the copy sheet is caused to contact the photoreceptor drum surface before the former is pressed against the latter by the grounded roller, in which method the copy paper sheet initially pressed against the photoreceptor drum surface by an electrically insulating roller is subsequently fed between the neighboring grounded roller and the photoreceptor surface. The above described conventional arrangement is effective for preventing generation of the white spots or absence of black tone in the developed image, since the surface of the insulating roller insulated from the copying apparatus housing is in an electrically isolated condition, with the surface potential of the former being varied according to the potential of the latent image formed portion when the latent image formed portion on the photoreceptor drum approaches the insulating roller as the photoreceptor drum rotates. In other words, in the above described conventional arrangement, transfer of the high potential part of the latent image formed portion is nearly completed at the portion of the insulating roller without generating the strong electric field between the copy paper sheet and the photoreceptor drum surface or the electrical discharge therebetween. In such case, however, there must be a certain distance between the insulating roller and the grounded roller, since if the distance between the two rollers is excessively small with respect to the copy paper sheet, the portion of the copy paper sheet whose reverse surface has electrical conductivity to a certain extent and which contacts the insulating roller is electrically connected to the grounded roller, thus giving rise to the absence of black tone in the developed image. However, separating the two rollers to a suitable extent from each other requires means for positively keeping the copy paper sheet in contact with the photoreceptor drum surface between the same rollers, because the charge on the photoreceptor drum surface after exposure is rather weak for positively holding the copy paper sheet on the photoreceptor drum surface, and especially when a photoreceptor drum of a small diameter is employed, it is extremely difficult to hold the copy paper sheet on the drum surface by the electrostatic attracting force alone.
In order to overcome disadvantages as described above, there are proposed transfer devices wherein a plurality of narrow belts of insulating nature such as rubber are directed around the insulating roller and the grounded roller as disclosed in parent application Ser. No. 500,171 or wherein a guide member is provided between the two rollers for preventing the copy paper sheet from coming off the photoreceptor surface. The former such device, however, still has a tendency such that the state under which the copy paper sheet contacts the photoreceptor surface tends to differ from one portion to another, thus resulting in fogging or blurring at the trailing edge of the developed image on the copy paper sheet, while in the latter such device, the guide member itself has no positive effect for holding the copy paper sheet on the photoreceptor drum surface, thus tending to cause jamming of the copy paper sheet.